Anti-cavitation mechanism for fluid driven systems



United States Patent Inventor Donald L. Bianchetta Coal City, IllinoisAppl. No. 716,745

Filed March 28, 1968 Patented Aug. 11, 1970 Assignee Caterpillar TractorCo.

Peoria, Illinois a Corp. of California ANTI-CAVITATION MECHANISM FORFLUID DRIVEN SYSTEMS 1 Claim, 1 Drawing Fig.

US. Cl 91/420, 91/443, 91/447 Int. Cl ..Fl5b 11/08, F 15b 13/42 Field ofSearch 91/420, 443, 447

References Cited UNITED STATES PATENTS Monroe Lankouski et a1 RoodThomas Brown et a1..

Parquet Primary Examiner Paul E. Maslousky Attorney- Fryer, Tjensvold,Feix, Phillips and Lempio ABSTRACT: A valve supplies driving fluid tothe head end of a hydraulic cylinder while presenting a variable flowrestriction to fluid being expelled from the rod end of the cylinder.The flow restriction is varied automatically in response to fluidpressure changes at the head end of the cylinder whereby the 1 rodcannot extend, in response to external load forces, at a rate fasterthan that provided for by the inflow of driving fluid at the head of thecylinder.

Patented Aug. 11 1910 3,523,490

5 INVENTOR DONALD L BIANCHETTA ATTORNEYS BY 't wf'g w 2M, 7% +4BACKGROUND OF. THE INVENTION This invention relates to fluid operatedmotors and more particularly to means for avoiding cavitation in suchsystems.

This invention was initially designed for use in conjunction with adouble acting hydraulic jack and will be described with referencethereto for purposes of example, it being understood that the inventionis also applicable to many other forms of fluid driven motor.

There are some working conditions under which external forces may pullor push the piston of a hydraulic jack in the same direction as theforce of the driving fluid acting thereon but at a greater rate thanthat provided for by the incoming driving fluid. This condition canoccur, for example, where the jack is used to lift the body of a dumptruck for unloading purposes. During the initial portion of the workingstroke, the jack works against the weight of the load in the truck bodybut at some point in the cycle the center of gravity of the load maypass over the pivot axis thereof. For the remainder of the stroke, theload tends to move the jack in the same direction as the driving fluidsupplied thereto. There are many other fluid operated systems wherein asimilar condition may occur. In any of these instances the phenomenon ofcavitation may result causing sudden uncontrolled movements, vibrations,possible damage and other undesirable effects. Damage to components fromoverly rapid travel and subsequent shock loads can also occur under thiscondition.

To avoid cavitation, it has heretofore been the practice to utilize amakeup valve or the like which directs the fluid being discharged fromone end of the jack back to the other end thereof to supplement theincoming driving fluid. This is not always a fully satisfactory meansfor preventing cavitation and under some circumstances a makeup valve isnot effective for this purpose, and because it will not control the rateof jack extension and resultant shock and possible damage when the jackreaches the limit of its extension.

To transfer fluid between the ends of the jack at a rate adequate toforestall cavitation, the makeup valve must sometimes be excessivelybulky and costly. In some extreme situations the size and cost of such avalve are prohibitive. This may be the case when the tendency for thejack piston to outrun its driving fluid is unusually pronounced becauseof a very large load. In instances where there is a very sizeabledifference in the cross-section area of the fluid volume on oppositesides of the piston, fluid is discharged at a rate which is insufficientto supplement the driving fluid. For these and other reasons, a

. conventional makeup valve, at least of practical size, cannot alwaysbe relied upon to prevent shock loads from damaging the fluid motor orto provide positive assurance against cavitation and its undesirableeffects.

SUMMARY OF THE INVENTION This invention is a fluid system which providespositive assurance against cavitation in a hydraulic cylinder or otherfluid motor by automatically limiting the rate at which fluid isdischarged to accommodate to the rate at which driving fluid is beingsupplied to the motor at any given time. The size of the variable flowpassage at the motor fluid discharge is determined by a valve memberwhich shifts in response to variations of pressure within the drivingfluid of the motor. In particular, the valve member shifts to decreasethe size of the discharge flow passage in response to a pressuredecrease within the driving fluid thereby maintaining adequatelybalanced fluid pressures throughout the motor irrespective of e theexternal forces acting thereon. The invention also prevents a dynamiceffect caused by the impact of load against the jack stops at the end ofthe stroke.

Accordingly, it is an object of this invention to provide. a

compact and a more reliable-m'eans'for avoiding ca-v-i-tation'in fluiddrivensystems. 1

The invention together with further objects and advantages thereof willbest be understood by reference to the following specification inconjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing is an axialsection view of an anti-cavitation valve according to the invention withcertain associated elements of the hydraulic system being shown inschematic form.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawing,there is shown a hydraulic.

I frame 16 at a pivot 17 which is situated somewhat forwardly from theback end of the body. The cylinder 11 is connected between the body 14and truck frame 16 forwardly from pivot 17 whereby extension of thecylinder lifts the forward end of the body to provide for a rear dumpingaction.

During the initial portion of the extension stroke of cylinder 11, thecylinder must work against the weight of the truck body 14 and contents.However with this construction, the center of gravity of the truck body14 and contents moves rearwardly as the body is raised and eventuallypasses to the rear of the pivot 17. At this point in the travel theweight of the "body and contents exerts a pull on the rod 13 of thecylinder source of fluid under pressure. Anti-cavitation valve 18 actsto restrict the discharge of hydraulic fluid from the rod end ofcylinder 11 in response to any pressure decrease in the driving fluid atthe head end of the cylinder.

For this purpose anti-cavitation valve 18 has a valve body 21 with fourhydraulic fluid ports 22, 23, 24 and 26. Port 23 is coupled to the headend of cylinder 11 by a conduit 27 while port 26 is coupled to the rodend of the cylinder by a second conduit 28. Ports 22 and 24 are coupledto a dump cylinder control valve 29 through separate conduits 31 and 32,respectively. Control valve 29 may be shifted by the operato'rto extendor contract the hydraulic cylinder 11 and thus to raise or lower thetruck body 14.

Control valve 29 has an inlet conduit 32 which receives a fluid, such asoil, under pressure from pump 19 and has an outlet conduit 33 to thefluid reservoir 34. The control valve 29 has a first position at whichhigh pressure fluid is supplied to port 22 of the anti-cavitation valve18 while port 24 thereof is vented to the reservoir 34 thereby extendingthe cylinder .11 as will hereinafter be discussed in more detail. At'asecond position of the control valve 29, anti-cavitation valve port24receives the hydraulic fluid under pressure while; port 22 is vented,thereby contracting the cylinder 11; while at the third, position of thecontrol valve both of the ports 22 and 24 are blocked to hold thecylinder 11 at any selected position. Valve 29 may also have a fourthfloat position at which conduits 31 and 32 are connected to provide forthe exchange of fluid from the head end to the rod end of cylinder 11.

To provide for the passage of driving fluid to the head end I ofcylinder 11 during'extension and for the discharge of fluid passage 36within valve body 21 communicates ports 22 and 23.

To provide a variable flow passage from the rod end of hydrauliccylinder 11 to control valve 29, valve body 21 has a stepped bore 37with an axially movable spool 38 therein. A passage 39 within valve body21 communicates port 26 with a large diameter portion 41 of bore 37. Asecond passage 42 within the valve body 21 connects port 24 with asecond smaller diameter portion 43 of bore 37. Spool 38 has a landsection 44 at the large diameter portion 41 of bore 37 and a smalldiameter portion 46 at bore portion 43. Thus by appropriate axialmovement of the spool 38, communication between valve body passages 42and 39 may be blocked by an edge 47 of land 44 or a flow passagetherebetween of variable size may be provided. To avoid overly extremesensitivity of the flow aperture between passages 42 and 39 to movementof spool 38, radially directed grooves 48 are provided in the spool atedge 47.

Thus when the cylinder 11 is undergoing an extension stroke edge 47provides a variable flow restriction between the rod end of the cylinderand control valve 29 with the degree of restriction being a function ofthe axial position of spool 38. The axial position of spool 38 is inturn determined by the fluid pressure at the head end of cylinder 11. Inparticular spool 38 is controlled by the fluid pressure in thepreviously described passage 36 which communicates with the head end ofcylinder 11.

For this purpose, an end 49 of the spool extends into an enlargedchamber region 51 at the end of bore 37 and carries a flange 52. Acompression spring 53 is disposed coaxially around spool end 49 withinbore region 51 and bears against flange 52 in a direction which tends tomove the spool 38 to close communication between valve passages 39 and52 at edge 47. Thus in the absence of a counterbalancing force on spool38, the flow passage from the rod end of cylinder 11 is closed. Toprovide such a counterbalancing force, region 51 of bore 37 iscommunicated with the passage 36 leading to the head end of cylinder 11by an additional passage 54. The fluid pressure at the head end of thecylinder 11 is thus transmitted to region 51 where it acts against aland 55 adjacent one end of bore region 51. A passage 56 in spool 38communicates bore section 43 with the end region of the bore oppositefrom the spring 53 to avoid trapping of fluid therein.

In operation, the hydraulic cylinder 11 is extended by adjusting controlvalve 29 to supply fluid under pressure from pump 19 to anti-cavitationvalve port 22. Such fluid is transmitted through passage 36, port 23 andconduit 27 to the head end of the cylinder to act against the piston 12therein. As the piston 12 and rod 13 advance, fluid is discharged fromthe rod end of the cylinder 11 through conduit 28, port 26, and passage39. The discharging fluid then passes spool edge 47 and enters passage42 and port 24 to be returned to the reservoir 34 through the controlvalve 29.

The size of the flow passage past edge 47 is determined by the axialposition of spool 38 and this in turn is a function of the fluidpressure at the head end of the cylinder ll as hereinbefore described.If the fluid pressure at the head end of the cylinder increases, thispressure increase is communicated to bore region 51 and acts againstland 55 of the spool to increase the flow passage past edge 47 andthereby provides for a correspondingly greater rate of discharge fromthe rod end of the cylinder. Conversely, if the fluid pressure at thehead end of the cylinder 11 decreases, due to a movement of the piston12 in excess of that produced by the driving fluid at the head end, thepressure decrease is communicated to region 51 whereby spring 53 maymove the spool 58 to increase the degree of flow restriction at edge 47.This prevents excessively fast travel of piston 12 and thereby avoidscavitation effects.

In the absence of further provisions, the above described structurewould block the flow passage to the rod end of the cylinder when controlvalve 29 is manipulated to effect a contraction stroke. To provide forthe supply of hydraulic fluid from ump 19 to the rod end ofthe c linder11 to contract the cylm er, a check valve 57 provides or directcommunication between valve body passages 39 and 42, bypassing edge 47,but limits direct flow to a direction from passage 42 to passage 39. Forthis purpose, the check valve 57 may include a valve member 58 whichseats against a valve seat 59 in a passage 61 extending between passages39 and 42. A spring 62 biases valve member 58 to the closed position.Where. as in the present instance. the valve member 58 extends acrossflow passage 39, an enlargement 63 is provided to avoid obstructionthereof. Accordingly direct flow between passages 39 and 42 is preventedduring the extension stroke of the cylinder 11 as indicated above butvalve 57 opens during the contraction stroke whereby fluid underpressure at port 24 may be transmitted to port 26 and thus to the rodend of the cylinder.

in the present example where the cylinder 11 dumps a truck body ashereinbefore described, there is not a serious problem with respect tocavitation during the contraction stroke of the cylinder in that theload has been released from the truck body 14 at this time. However inother instances where there is a cavitation problem on both theextension and contraction strokes, it will be apparent that a similarvariable flow restriction may be provided in the flow passage to thehead end of the cylinder.

lclaim:

1. In a fluid system for supplying driving fluid to a cylinder of theclass having a piston therein and having an axially directed extensibleand contractible rod coupled to said piston and wherein said cylinderhas a fluid port at each end thereof, the combination comprising:

a source of fluid under pressure,

a control valve coupled between said fluid source and said cylinder fordirecting fluid to one end of said cylinder while discharging fluid fromthe other end thereof, and

an anti-cavitation valve connected to said other end of said cylinderand forming a flow passage for said fluid discharging therefrom, saidflow passage being isolated from said one end of said motor to transmitall of said discharging fluid away therefrom, said anti-cavitation valvehaving an element at said passage which is movable to vary the flowaperture there-through in an incremental manner from a fully closedposition to a fully open position, said element being responsive toincremental changes of the fluid pressure at said one end of saidcylinder, wherein said control valve has a first setting at which fluidis directed to said one end of said cylinder and at which fluid isreceived from said other end of said cylinder through saidanti-cavitation valve for return to said source, said control valvehaving a second setting at which fluid is directed to said other end ofsaid cylinder and received from said one end thereof, and

a check valve defining an additional passage between said control valveand said other end of said cylinder which additional passage bypassessaid movable element, said check valve being capable of transmittingfluid from said control valve to said other end of said cylinder andblocking flow in a reverse direction through said check valve.

